Optical fibers have been widely used for the propagation of optical signals, especially to provide high speed communications links. Optical links using fiber optics have many advantages compared to electrical links: large bandwidth, high noise immunity, reduced power dissipation and minimal crosstalk. Optical signals carried by optical fibers are processed by a wide variety of optical and optoelectronic devices, including integrated circuits. Optical communications signals in optical fibers are typically in the 1.3 μm and 1.55 μm infrared wavelength bands. Optoelectronic integrated circuits made of silicon are highly desirable since they can be fabricated at low cost in the same foundries used to make VLSI integrated circuits. The optical properties of silicon are well suited for the transmission of optical signals, due to its transparency in the infrared wavelength bands of 1.3 μm and 1.55 μm and its high refractive index. As a result, low loss planar silicon optical waveguides have been successfully built in silicon integrated circuits.
Optical signals traveling in optical fiber frequently need to be coupled to optoelectronic circuits and this can be done through a variety of known techniques and devices. Once an optical signal is on a chip, it can be processed either as an optical signal or converted to an electronic signal for further processing.
The flat end of an optical fiber can be directly connected to the edge of an integrated circuit, so an optical signal can be coupled to a flat end of an integrated waveguide, but the fiber and the waveguide have different cross sectional geometries and are very different in size.
An optical signal in a fiber can be coupled to a waveguide through the top surface of an integrated circuit using a waveguide grating coupler, which is more effective as a fiber to chip connector. But there are difficulties in connecting an optical signal from a fiber to a waveguide using a waveguide grating coupler, due to differences in: cross sectional geometry, the number of optical modes and polarization characteristics.
A circular cross section with a core diameter of less than ten microns is typical for a single mode fiber (SMF). A nanophotonic waveguide is typically rectangular in cross section and can be substantially smaller than one micron in both cross sectional dimensions. A waveguide grating coupler can be designed to make a usable connection between a fiber and a waveguide, even with their inherently different cross sectional and size characteristics.
A typical SMF fiber can have a gaussian mode profile, with most of the power concentrated in the center of the light beam. A waveguide can be designed to support a gaussian type of mode profile with most of the power in the center of the waveguide. A waveguide grating coupler can provide a usable connection between a fiber and a waveguide, if they have compatible gaussian mode profiles.
A waveguide grating coupler can be designed to connect a fiber to a waveguide, even when a fiber and a waveguide have very different cross sectional, size and mode profile characteristics. But connecting an optical signal from a fiber to a waveguide can cause significant signal loss and distortion due to the typically different and incompatible polarization characteristics.
An optical beam traveling in a single mode fiber (SMF) with circular cross section can typically be decomposed over an arbitrary basis of two orthogonal polarizations. These two orthogonal polarizations have approximately the same propagation constant and approximately the same group velocity in an optical fiber. Some refer to these two modes as a single mode with two polarization components. Within this discussion of the present invention, the two orthogonal polarizations are referred to as two modes.
In theory and under ideal conditions, there is no exchange of power between the orthogonal polarizations in an SMF fiber. If an optical signal is directed into only one polarization, then all the power should remain in that polarization. But in actual practice, imperfections or strains in the fiber cause random power transfer between the two polarizations. The total power is thus divided between the two polarizations, and this may not be a problem in some applications, but in many situations, this can be a major problem. In some cases, there can be a great deal of fluctuation and power transfer between the two polarizations. Such random fluctuations can cause the power delivered on one polarization, to be close to zero, which would result in considerable loss of signal, if only that polarization is being received by a waveguide on a chip.
Similarly, two orthogonal polarization modes are present in standard forms of polarization maintaining fibers. These two modes have sufficiently different phase and group velocities to prevent light from coupling back and forth between the two modes. But a typical waveguide cannot usually propagate both of the modes, which would result in some signal loss in such a fiber to waveguide connection.
Typical integrated optical waveguides have a different type of modal configuration, where there are two primary types of modes: the transverse electric (TE) and the transverse magnetic (TM), which describe which field of the mode is oriented purely transversely to the direction of propagation. This is strictly true only for two dimensional ideal waveguides, however this naming convention is also used for real world three dimensional waveguides, which are only approximately TE or TM. Future references herein will make the common assumption that quasi-TE or quasi-TM modes are understood as TE or TM modes. A waveguide grating coupler can be designed to connect one polarization of an optical signal from a fiber to only one of the TE or TM modes. The other orthogonal polarization of the optical signal in the fiber would not be connected to the waveguide and any information transported by that polarization would be lost.
As a result of the differences in polarization characteristics between fibers and waveguides, it has been difficult to connect optical signals from one to the other.